An escalating controversy over the alleged shooting of Iraqi civilians by a U.S. security firm has triggered the strongest challenge yet to legal immunity for some foreigners in Iraq, while providing a rare rallying cry for the country's polarized factions.

Iraqi government statements have been contradictory on the Sept. 16 gunfire, in which Blackwater USA, based in North Carolina, is accused of having killed Iraqi civilians while escorting an American diplomatic convoy in Baghdad. They range from threats to prosecute Blackwater to promises not to expel the firm from Iraq.

But the government of Prime Minister Nouri al-Maliki has managed to galvanize broad-based opposition to an order issued in the waning days of direct American rule in Iraq that lays out broad immunity from criminal prosecution for U.S. diplomats, troops and private contractors operating in Iraq.

It is known as Order 17, issued by the U.S. Coalition Provisional Authority in 2004. Iraqi officials have long chafed at the law, viewing it as an encroachment on Iraqi sovereignty. But until now, no serious effort has been made to revise it.

The central government, unpopular on the streets and worried about being marginalized, appears to be using the Blackwater crisis to counter U.S. criticism that it is ineffective and to show ordinary Iraqis that it can stand up to Washington.